This invention relates to a discharge lamp lighting circuit which reduces the number of parts and costs by improving the configuration of a DC power supply circuit and a DC-AC conversion circuit making up the parts of the discharge lamp lighting circuit.
The configuration of a lighting circuit of a discharge lamp, such as a metal halide lamp, comprising a DC power supply circuit, a DC-AC conversion circuit, and a starter circuit is known. For example, in the configuration wherein a DC-DC converter is used as a DC power supply circuit and a full-bridge type circuit comprising two pairs of semiconductor switch elements for performing switching control and a driver circuit thereof are used for a DC-AC conversion circuit, the positive-polarity voltage (positive voltage) output by the DC-DC converter is converted into rectangular-wave voltage in the full-bridge type circuit, then this voltage is supplied to a discharge lamp.
To light a discharge lamp more reliably, the voltage applied to the discharge lamp needs to be set to a reasonably high voltage (overcurrent voltage) temporarily before the discharge lamp lights up. The reason is as follows: When a start pulse generated by a starter circuit is applied to the discharge lamp and the discharge lamp breaks down, the tube voltage of the discharge lamp lowers, so that charges of a smoothing capacitor in a DC power supply circuit or charges of a capacitor in a current auxiliary circuit (for example, refer to JP-A-9-223591) provided at a later stage of the DC power supply circuit become an electric current to the discharge lamp and transition to ark discharge can be accomplished reliably.
By the way, to light a plurality of discharge lamps by a lighting circuit in the related art, a DC power supply circuit and a DC-AC conversion circuit of full-bridge type configuration become necessary for each discharge lamp and the above-mentioned current auxiliary circuit becomes necessary at a later stage of each DC power supply circuit, thus the circuit configuration is complicated; this is a problem.
For example, to use a discharge lamp as a light source of a car""s front light, if a front light is attached to each of the left and the right of the front of the vehicle, two left and right discharge lamps and their respective lighting circuits become necessary. To adopt a configuration wherein high and low beams are provided by separate discharge lamps (so-called four-light illumination), two left and two right discharge lamps and their respective lighting circuits are required.
It is therefore an object of the invention to simplify the circuit configuration of a lighting circuit of a plurality of discharge lamps and reduce the costs of the lighting circuit.
To the end, according to the invention, there is provided a discharge lamp lighting circuit comprising a DC power supply circuit for receiving DC input voltage and outputting any desired DC voltage, a DC-AC conversion circuit for converting the output voltage of the DC power supply circuit into AC voltage and then supplying the AC voltage to a plurality of discharge lamps, a detection circuit for detecting voltage or current related to each discharge lamp, and a control circuit for controlling voltage, current, or supply power of each discharge lamp in response to a detection signal from the detection circuit. In the discharge lamp lighting circuit,
(a) positive-polarity voltage and negative-polarity voltage output separately from two output terminals of the DC power supply circuit are sent to the DC-AC conversion circuit;
(b) two pairs of switch elements provided in the DC-AC conversion circuit to switch the output voltages of the DC power supply circuit form a full-bridge type circuit configuration and AC voltage generated by alternately operating the switch elements in pairs by drive circuits of the switch elements is supplied to each discharge lamp; and
(c) to light one of the discharge lamps, the state of each of the switch elements is fixed so that the polarity of the voltage supplied from the DC-AC conversion circuit to the discharge lamp before the discharge lamp is started is defined as either positive or negative polarity and the switch elements are alternately operated after the discharge lamp is lighted.
Therefore, according to the invention, for a plurality of discharge lamps, the two pairs of switch elements are provided in the DC-AC conversion circuit to form a full-bridge type circuit configuration and drive control is performed so as to alternately operate the switch elements. Thus, the circuit configuration is simplified and moreover, the polarity of the voltage supplied to the discharge lamp before the discharge lamp is lighted is fixed to either polarity, whereby the discharge lamp can be well lighted.